Such a monitoring system has been described, for example, in U.S. Pat. No. 4,193,023 to Robert Buck and Gerd Marhofer, as well as in various earlier patents cited therein. Conventionally, e.g. as disclosed in U.S. Pat. No. 3,747,010, the oscillator output is rectified and intergrated by a blocking condenser and a bypass diode in the input of a transistor inserted in a charging path of a storage capacitor which is shunted by a relatively high-ohmic discharge resistance so as to accumulate a charge of a maginitude depending on the oscillator amplitude. When that charge reaches a certain level, an electronic switch such as a thyristor is triggered to energize a load for signaling the occurrence of the event to be monitored.
While this type of monitoring system operates generally satisfactorily, it may still be susceptible to an untimely triggering of its electronic switch by externally induced transient voltages charging the storage capacitor above the critical level. This is due to the fact that the charging path of the storage capacitor has a relatively short time constant, in comparison with that of its discharging path, whereby even a small number of rapidly recurring spurious pulses in the transistor input will cause a significant charge increase.